Method of producing castings with one or more internal passages



#3 e -sheet 1' E. R. GADD' HAL METHOD OF aonucmc. OASTINGS WITH ONE ORMORE INTERNAL PAssAcE s Filed Nov; 1,1954

E. R- GADD ET AL July 29, 195 2,844,855 0 METHOD OF PRODUCING CASTINGSWITH ONE OR MORE INTERNAL PASSAGES 7 Filed Nov. 1. 1954 2 Sheets-Sheet 2OOOOOOOOOOOOO United States Patent METHOD OF PRODUCING CASTINGS WITH ONEOR MORE INTERNAL PASSAGES Application November 1, 1954, Serial No.466,136

Claims priority, application Great Britain November 5, 1953 Claims. (Cl.'22-204) This invention relates to the production of metal castings withone or more internal passages, and is concerned primarily with theproduction of metal blades for turbines, compressors and otherfluid-flow power conversion machines, in which blades it is necessary ordesirable to provide one or more internal passages for the flow of acooling or a heating medium. The invention is not, however, restrictedto the production of such blades but may be employed generally for theproduction of any casting in which one or more internal passages arerequired.

According to the invention, a method of producing a metal casting havingat least one internal passage comprises casting the casting in a mouldin which is supported a metal tube forming a core at the position atwhich said passage is required.

According to a feature of the invention, the method may include the stepof cooling the tube while the casting metal is being run into the mouldand/or immediately after the casting metal has been run into the mould.

Such cooling may be effected by inserting a removable core of heatabsorbing substance within the tube, the core being removed during orafter the casting process, for example, in the case of a solidsubstance, by fusion or withdrawal. Alternatively, the cooling may beeifected bypassing a liquid coolant such as a molten metal or a gaseouscoolant such as air through the tube.

By way of explanation, it is to be noted that when the tube is made of ametal having a melting temperature lower than the temperature of thecasting metal being run into the mould it may be necessary to cool thetube in order to prevent its collapse, and the cooling by passing afluid coolant through the tube should in this case be regulated so thatthe internal temperature of the tube does not exceed a value at whichthis might happen.

The cooling may, however, be regulated so that an outer superficiallayer of the tube material is fused by heat from the metal run into themould and promotes bonding of the tube with the metal of the casting.

In some cases where the tube is made of a metal having alower meltingtemperature than the casting metal being run into the mould it may besufficient, instead of cooling the tube, to insert a removable core inthe tube to support the wall of the tube against collapse when thecasting metal is run into the mould, the core being subsequently removedafter the casting metal has cooled below the melting temperature of themetal of the tube. Where, on the other hand, the tube is made of a metalhaving a melting temperature higher than thetemperature of the castingmetal being run into the mould, such a tube may, according to anotherfeature of the invention, be internally heated while the casting metalis being run into the mould, this to prevent excessive chilling of thecasting metal coming into contact with the tube. 70 Thus, the tube maybe internally heated by inserting within the tube a removable corecomprising electrical heat- 2v ing means and supplying electric currentto the heating means, the core comprising the heating means beingremoved during or after the casting process.

According to yet another feature of the invention, the tube may becoated externally with a material promoting bonding of the cast metalwith the metal of the tube. Thus, for example, in a high temperaturecasting process using, say, a Nimonic alloy as the casting metal, theexternal surface of the tube may be coated with a metal which is moreresistant to oxidation than the metal of the tube itself. While in a lowtemperature casting proc ess using, say, an aluminium alloy as thecasting metal, the external surface of the tube may be coated with aflux film, or in a further example, a tube which is composed of amaterial having a melting temperature as high as or higher than thetemperature of the casting metal being run into the mould may be coatedexternally with another metal having a melting temperature lower thanthe temperature of the metal to be run into the mould so that thecoating fuses and promotes bonding as stated above. 7

According to yet another feature of the invention, the method mayinclude the step of removing, by chemical, mechanical or electricalmeans after the casting process, at least part of the metal of the tubefrom the interior of the tube in order to enlarge the internal passagein' the casting. Thus, the bore of the tube may be enlarged or the Wholeof the tube material may be removed.

According to yet another feature of the invention, the method mayinclude removing by chemical, mechanical or electrical means not onlythe whole of the metal of the tube, but also part of the cast metalsurrounding the tube so as further to enlarge the bore. Furthermore, thetube may itself be provided on its external surface with raised membersin the form of fins or gills, so that when removed by chemical action abore is left having fins or gills which are a negative counterpart ofthose on the tube.

Methods in accordance with the invention for the production of turbineblades will now be described by way of example, with reference to theaccompanying drawings, whereof:

Figure 1 is a cross sectional elevation of a mould for casting a segmentof nozzle guide blading for a gas turbine engine, a number .of metaltubes being supported in the mould to form cores at positions at whichinternal passages are required in the baldes of the segment,

Figure 2 is a cross-section on line 22 of Figure l,

v Figure 3 is a partial view in cross-section of a mould showing a tubesupported therein and containing a core comprising an electrical heatingelement,

Figure 4 is a partial view in cross-section of a tube for casting into acasting in accordance with the invention to form an internal passage inthe casting, the tube having external raised members,

Figures 5 and 6 show examples in cross-section of internal passagesproduced in accordance with the invention, the metal of the cast-in tubehaving been removed by chemical action in each case,

Figure 7 shows a blade produced in accordance with the presentinvention,

Figure 8 is a cross section of another blade produced in accordance withthe present invention,

Figure 9 is a cross-section on line 99 of Figure 8,

v Figure 10 is a cross-section on line 1 0-10 of Figure 8,

Figure 13 is an elevation of yet another blade produced in accordancewith the, present invention,

Figure 14 is a view of Figure 13 in the direction of arrow 14,

Figures 11 and 12 are elevations of a pair of sheet metal parts whichtogether form the tubes which are cast into the blade'shown in Figures13 and 14, and Fig-- ure is a perspective view of a pattern plate usedin another method in accordance with the invention.

Referring to Figures 1 and 2, the segment of nozzle guide bladingcomprises four blades joined by inner and outer shroud ring segments.

The segment is cast from a cobalt-base alloy of the kind known by thetrade name Stellite, each blade having a number of embedded stainlesssteel tubes 10 of continuous or butt-jointed section or any otherconvenient construction running longitudinally through the blade and isproduced in a manner which will now be described.

The segment of blading is made by a lost-wax precision casting method.The tubes 10, which may first be chromium plated to increase theirresistance to oxidation, are embedded in their correct positions in awax pattern of the blading segment and are of additional length so thattheir ends project well clear of the pattern. The pattern comprises apart corresponding to the segment of blading and a part corresponding tothe pouring gate 11. The pattern is then surrounded with investmentmaterial to form a mould while leaving the ends of the tubes projectingthrough the investment material. After the investment has set, the mouldis heated sufficiently to expel the wax from the mould and is then firedat a temperature between 950 C. and 1000" C. which is a sufficientlyhigh temperature to stabilize the investment material. When such firingis complete the molten casting alloy which is at a temperature between1500 C. and 1550 C., is run directly into the mould through the gate 11while the mould is at a temperature between 600 C. to 700 C.

The coating of chromium on the outside of the tubes 10, increases theresistance of the outer surfaces of the tubes to oxidation during firingof the mould and by the molten casting alloy when this is run into themould. By preventing or reducing the formation of an oxide film, thechromium coating promotes the bonding of the cast metal to the tubes.Stainless steel has a melting temperature in the range of 1425 C. to1470 C. which is slightly less than the temperature of the moltencasting alloy. Unless the tubes 10 are of small bore'and wall thickness,however, it is found that they do not collapse due to melting. Ifdesired or necessary, a core of refractory material is inserted in eachtube to support the tube against possible collapse when the castingmetal is run into the mould, however, these cores of refractory materialbeing removed when the metal has cooled or after it has finally set. Therefractory material used may be any of those later mentioned withreference to Figure 3.

The method just described may also be used for embedding stainless steeltubes in a segment of turbine nozzle guide blading cast fromnickel-chromium heat resisting alloys of the kind known by the tradename Nimonic, but in this case, owing to the higher casting temperatureof the alloy, namely 1550 C. to 1600 C., it is preferred to cool thetubes, especially any tubes of small bore and wall thickness, by passinga flow of cooling air through them.

To this end, referring again to Figures 1 and 2, the wax patternpreviously described is provided with a part attached to one end of thetubes 10 in each group of tubes 10 in the four pattern partscorresponding to the blades of the segment to produce air outlets 12 inthe mould leading from the groups of tubes to the outside of the mould.The other ends of the tubes are fixed into a tube plate 13 prior to thetubes being embedded in the wax pattern, the tube plate 13 being keptoutside the investment material 20 when the pattern is invested to formthe mould. When the mould is ready for casting a manifold 14 having apipe 15 connected to a supply of cooling air under pressure is securedto the tube plate so that the ends of the tubes opening in the tubeplate communicate with the interior of the manifold. The mould is thenpreheated to between 600 C. and 700 C. if the temperature of the mouldafter the firing has dropped below. this range, and the casting alloyrun into the mould 4 through the gate 11. The cooling air is turned onbetween the commencement of pouring and a time immediately after thepouring has been completed, and the cooling air flows from the manifoldthrough each of the tubes and out of the outlets 12. When thetemperature of the cast metal has fallen sufficiently to ensure that thetubes will not collapse under the heat of the cast metal, the air supplyis turned off.

The cooling air supply is regulated so that the internal temperature ofthe tubes never exceeds a value at which the tubes might collapse due tomelting.

Instead of passing cooling air through the tubes, molten metal at atemperature lower than the melting temperature of the stainless steeltubes 10 may be used. For example, molten lead, aluminum, brass orcopper may be used as the coolant, the selection made depending on thedegree of cooling required to ensure that the tubes do not collapse.

Whatever the cooling medium used, the cooling can also be regulated sothat an outer superficial layer of the tubes 10 are fused by heat fromthe nimonic casting alloy run into the mould. This promotes bondingbetween the cast metal and the tubes. If the melting temperature of themetal of the tubes is, in another case to that being described, higherthan the temperature of the molten alloy, a similar effect can beachieved by coating the tubes externally with a metal having a lowermelting temperature than the temperature of the molten alloy so that themetal coating fuses when it comes into contact with the molten alloy andpromotes bonding. For example, mild steel tubes which are to be embeddedin a steel alloy of the nickel-chromium-tungsten type may becopper-plated externally to provide them with an external coating whichwill fuse on contact with the molten casting alloy.

Another method, according to the invention, for making a blade by theshell moulding process, this process being particularly convenient forcasting separate blades, will now be described. In this case, a pair ofmetal pattern plates one of which is shown in Figure 15, are made withraised parts 21 corresponding to throughways in the mould shell requiredfor the passage of the tubes which are to be embedded in the blade. Themould shells are made in the usual manner by dumping a mixture of sandand thermosetting resin upon the heated pattern plates and thencompleting the curing of the shells in an oven. The pattern plate shownin Figure 15 is a pattern of one-half of the blade and certain detailssuch as dowels and ejector pins are not shown. This pattern plate isused to produce one-half of the shell mould, the other half beingproduced using a pattern plate corresponding to the other half of theblade. After removal from the pattern plates the shells are assembledtogether and the tubes are inserted between the mould parts, the tubesbeing supported between the mould parts with the ends of the tubesprojecting one through each of said throughways to the outside of themould. The tubes are made of mild steel in this case, since nopre-heating such as would give rise to surface oxidation of the tubes isrequired. A cooling air supply, if necessary, is arranged for the tubesas previously described, with reference to Figure l, and the castingmetal is poured into the mould. After removing the casting from themould the excess lengths of tube are trimmed off, or the blade may firstbe subjected to chemical action by passing a suitable acid through oneor more of the tubes to increase their bore or to remove them entirely.

Alternatively, the bores may be enlarged by drilling or by an ultrasonicmachining process or by a spark or are machining process, the originalbore constituting a pilot hole for an inserted tool. A bore may also beenlarged by using a high frequency magnetic field to melt out a tube ifthe electrical properties of the metal of the tube are different fromthose of the cast metal. As well as removing the metal of the tubes,part of the surrounding cast metal may also be removed if desired so asfurther to enlarge the bore of the internal passages.

By way of further example, a blade 26 (Figure 13) produced by any of themethods describedhas a cast-in insert composed of two sheets of metal23, 24 (see Figures 11 and 12), formed by apressing operation toconstitute component parts of a manifold tube system generally indicatedat 25 in Figure 13,'-this comprising a header 27 and four branch tubes30, 31, 32 and 33. The metal sheets23, 24 are thus formed with channels36 which are then superposed face to face, the sheets 23, 24 beingjoined by spot welding along the portions 37 between the channels 36 andaround the' edges of the sheets or in any other-convenient manner. Theheader part 27 ofv the tube system is arranged within the root portion40 of the blade and the branch tubes 30, 31, 32 and 33 extendlongitudinally through Working portion 41 of the blade. The jointsurfaces between the portions 37 are curved in conformity with thecurvature of the blade as shown in Figure 14 so that the portions 37form a central'fin interconnecting the tubes 30, 31, 32 and 33 andextending into the leading and trailing edges 50 and 51 of the blade,such an arrangement being particularly advantageous when the tube systemis pressed from a metal of higher conductivity than the cast metal sinceit greatly assists the conduction of heat to or from the leading andtrailing edges of the blade which if sharp, cannot always be adequatelycooled or heated by a medium passing through internal passages in theblade. As shown in Figures 11 and 12, registering holes 52 are providedin the portions 37 between the channels, through which the castmetal'flows. This helps in bonding the manifold tube system into thecast metal of the blade.

' The manner of forming a plurality of tubes .to be cast into a castingin accordance with the invention, which has just been described may, itwill be appreciated, he adopted in the case of a single tube. Forexample, the tubes in Figures 1 and 2 may each be composed of a pair ofsheet metal parts each having a channel such as 36 constituting one partof the wall of. the tube and a flange part corresponding to portions37on each side and running the whole length of the channel, the flangeparts having their long edges directed away from one another, and thesheet metal parts being joined together with the flange parts of oneoverlying the flange parts of the other, to form the tube.

It will be noted that in the manifold tube system described withreference to Figures 11 to 14, there are five openings to the outside ofthe casting, namely an inlet opening 60 and four outlet openings 61.This may also be achieved by embedding a branched tube in a casting byany of the methods described. This is illustrated in Figure 7 whichshows a blade 70 having a cast in metal tube 71 extending throughthe'root portion 72 of the blade, this tube having four branches 73which extend from the portion of the tube transversing the root portion72 at right angles thereto, and pass longitudinally through the workingportion 74 of the blade. The projecting parts of the tubes are cut offwhen finish-machining the blade.

Furthermore, the portion of the tube 71 in the root portion 72 could forexample, be arranged along one side of the root portion, this portionbeing subsequently removed by any of the mechanical, electrical orchemical means previously referred to to leave a header 74 (see Figure8) in the blade root with which chamber the branches 73 communicate. Thebranches of the tube are in this case curved as at 75 to pass from thechamber 74 into the working portion of the blade, and are suitablyvaried as to cross-sectional area and/or cross-sectional shape alongtheir length so that they are more easily accommodated in the workingportion of the blade both at the centre and towards the leading andtrailingedges of the blade. This feature is shown in Figures 9 and 10from which it will be seen that the central branch 73 changes fromcircular cross-section at the root-end 72 of the blade to a narrowsector shaped cross-section 6 at the tip end, where in view of thethinness of the blade; the cast metal could not easily accommodate thetube if its cross-sectional shape and cross-sectional area were the sameas at the root end. In a similar way, the branches adjacent the leadingand trailing edges of the blade change from circular cross-section atthe root end to a flattened somewhat triangular shape in cross-sectionat thetip end.

Where tubes cast into a casting by any of the methods hereinbeforedescribed are to *be subsequently removed to leave internal passages inthe casting of larger bore than the tubes, the tubes may be provided ontheir external surfaces with raised members such as in Figure 4 so thatwhen the metal of the tube is removed the passage, being in shape anegative of the external shape of the tube, has recesses 81 (Figure 5)corresponding to the raised members 80. In this manner, internalpassages in cast blades may be provided with such recesses as materiallyincrease the heat exchange surface of the passage. The raised members 80may take any desired form for example, plane ribs, flanges or gills, orthey may be helically arranged around the outside of the tube like theturns of a multi-start thread. A tube having raised members in the lastmentioned form gives rise to an internal passage as shown in Figure 6when it is removed.

Such tubes may for example, be removed by chemical attack. When thismethod of removal is to be employed,- the tube is of course made from ametal which is removable by the selected reagent to which the cast metalshould be immune.

In general, the metal tubes may be made of mild steel, stainless steel,nickel-chromium heat-resisting alloy, copper or silver, and the castmetal of the blade may be aluminium alloy, bronze, mild steel, stainlesssteel, or a heat-resisting nickel-chromium or cobalt base alloy, thematerials being appropriately chosen according to the intended workingtemperature of the blade and the stresses which it will be called uponto bear during use.

Where aluminium alloy is to be used as the casting alloy, tubes madeofany of the metals of alloys mentioned above have a melting temperaturehigher than the temperature of the molten alloy run into the mould. Inthis case, in order to prevent chilling of the molten alloy coming intocontact with the tubes the tubes may be heated internally, for example,by inserting into each before the molten metal is poured, a corecomprising an electrical heating element 16 (see Figure 3) enclosed inan insulating heat resisting refractory material 17, and connecting theelement to a source of electric supply. As soon as the molten alloy hasbeen poured and the whole assumed an even temperature the current supplyis switched off and the cores then removed either immediately, or afterthe cast metal has set.

To promote bonding of the aluminium alloy with the tubes the tubes arecoated externally with flux before the alloy is poured. The refractorylayer 17 on the electric heating element may be composed of silica,alumina, magnesia, or zirconia, all of which may be deposited on theheating element by electrophoresis. Aluminium blades having internalpassages are used for example as compressor inlet guide blades or vanes,the internal passages serving for heating purposes to avoid icing of theblades.

We claim:

1. A method for producing a metal casting having a group of internalpassages, which method comprises shaping each of a pair of metal sheetsto form the sheet into a series of passage wall parts of passagescorresponding to and grouped in the relation required for the passagesin the casting with a flange part on each side of and running the wholelength of each such wall part, joining said sheets together with theflange parts of one overlying the flange parts of the other to form aunitary core having passages corresponding to and grouped in therelation required for the passages in the casting, supporting said corein a mould of the required casting with the passages in the core at theposition required for the passages in the casting and then casting thecasting in the mould.

2. A method as claimed in claim 1, comprising making the core from sheetmetal having a melting temperature higher than the temperature of thecasting metal being run into the mould, and the core is internallyheated while the casting metal is being run into the mould.

3. A method as claimed in claim 2, comprising internally heating thecore by inserting within each of the passages of the core a removableplug comprising electrical heating means, and supplying electric currentto the heating means.

4. A method as claimed in claim 1, comprising supporting the walls ofthe passages in said core against collapse when the casting metal is runinto the mould by inserting a removable plug in each of the passages ofsaid core before running the casting metal into the mould.

5. A method as claimed in claim 1, comprising, before running thecasting metal into the mould, coating said core externally with amaterial which promotes bonding of the cast metal with the metal of thecore.

6. A method as claimed in claim 5, wherein said coating is of a metalwhich is more resistant to oxidation than the metal of said core.

7. A method as claimed in claim 5, wherein the casting process is a lowtemperature casting process, and said coating is of flux.

8. A method as claimed in claim 5, comprising making the core of sheetmetal parts of a metal having a melting temperature at least as high asthe temperature of the casting metal to be run into the mould, saidcoating being of a metal having a melting temperature lower than thetemperature of the casting metal to be run into the mould.

9. A method as claimed in claim 1, further comprising embedding saidcore in the desired position in a wax pattern of the casting to beproduced, with the ends of said core projecting well clear of thepattern, investing the pattern with investment material to form a mouldwhile leaving the ends of the core projecting through the investmentmaterial, allowing the investment material to set, heating the investedpattern to expel the wax from the mould, firing the mould to stabilizethe investment material, and then running the casting metal directlyinto the hot mould.

10. A method as claimed in claim 1, for producing a blade for a fluidflow machine, said blade having a working portion with leading andtrailing edges, a root portion and a group of internal passages runninglongitudinally of the working portion, the method comprising making saidsheet metal parts of a metal having a higher thermal conductivity thanthe casting metal with longitudinal edge portions which extendrespectively into leading and trailing edge portions of the workingportion of the blade.

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